The objective of this project is to determine the structure of polyoma virus and investigate the relationship between structure and function in the assembly of the virus particles. X-ray crystallography is being used to derive the three dimensional structure of the complete infectious polyoma virion and its nucleic-acid-free capsid. Comparison of these structures should elucidate the role of the capsid protein in the encapsulation of the viral DNA and yield structural information concerning the viral nucleohistone, a functionally intact "mini-chromosome." In the coming year x-ray data will be collected from the capsid crystals to 8 angstrom units resolution using the oscillation method, and the search for isomorphous heavy atom derivatives at 22 angstrom units will be continued. Interpretation of the 22 angstrom units data from the capsid crystals should be completed shortly. Also, as soon as facialites for handling low risk viruses are completed at Brandeis, data will be collected from crystals of the complete infectious virion to 22 angstrom units resolution.